1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
Active-matrix type liquid crystal display device including transistors as active elements provided at each pixel are capable of displaying high-definition and high-quality images, so that those are used often for display devices of liquid crystal television sets, portable devices, and the like. Among those active-matrix type liquid crystal display devices, those using polycrystalline thin film transistors (referred to as “poly-Si TFT” hereinafter) for the transistors are used especially for liquid crystal display devices of small pixel size, because of the following reasons. That is: with such type, the transistors have high current drive capability, so that the size of the transistor to be provided to each pixel can be reduced; a circuit for generating signals to be supplied to each pixel can be fabricated on a same substrate where each pixel is formed; etc.
FIG. 22 is a circuit block diagram showing an equivalent circuit for one pixel of a liquid crystal display device using poly-Si TFT. Explanations will be provided hereinafter by referring to this drawing.
In the drawing, a transistor Tr1 is provided to each pixel. A pixel capacitor Cpix connected to a source electrode of the transistor Tr1 is formed by a pixel electrode, a counter electrode, and a liquid crystal layer sandwiched therebetween. Further, a holding capacitor Cst is connected to the source electrode of the transistor Tr1. A gate electrode of the transistor Tr1 is connected to a gate line Gn, and a drain electrode of the transistor Tr1 is connected to a data line Dm.
In a period for displaying an image for one screen of the liquid crystal display device, the transistor Tr1 operates to keep video signals that are written to the pixel capacitor Cpix and the holding capacitor Cst in most of that period. It is possible to obtain a fine picture quality with less flicker and crosstalk, if voltages of the pixel capacitor Cpix and the holding capacitor Cst do not fluctuate during that holding period.
Recently, there has been a strong demand on the market for achieving performances such as high definition and high luminance in the display devices. Accordingly, pixel pitches of the liquid crystal display devices have become smaller, and the luminance of the backlights as light sources has been increased. The luminance of the liquid crystal display device depends almost on the luminance of the backlight and the transmittance of the pixels of the liquid crystal display device, and the transmittance of the pixels change greatly according to the numerical aperture. When the pixel pitch becomes smaller because of achieving high definition, the numerical aperture naturally becomes smaller as well. In addition, values of the pixel capacitor and the holding capacitor also become smaller. Further, leak currents of the transistors are increased depending on the amount of light to be irradiated to the transistors. Therefore, in the high-definition and high-luminance liquid crystal display device, the voltages of the pixel capacitor and the holding capacitor become fluctuated during the holding period, thereby generating flicker and crosstalk.
Especially, in a case of a liquid crystal display device using a top-gate type poly-Si TFT, the light from the backlight is irradiated directly to the channel part of the transistor. Thus, a light leak current thereof becomes larger than that of a liquid crystal display device using an amorphous silicon thin film transistor (referred to as “a-Si TFT” hereinafter) which is typically a bottom-gate type. This results in having more serious issues.
Further, crosstalk is largely affected not only by the extent of the leak current of the transistor but also by “dependency of the leak current on a voltage Vds between the source and the drain”. Furthermore, provided that a potential of the data line Dm is Vdata and a voltage of the pixel capacitor Cpix is Vpix, Vds is a function of Vdata and Vpix. Thus, the voltage between the source and drain of the transistors of each pixel fluctuates largely depending on the luminance of a signal written to each pixel that is connected to the common data line. Therefore, the leak current of the transistors is to change largely. As a result, when a specific pattern is displayed, pixels that are not displaying the pattern are to be affected, thereby generating crosstalk.
Japanese Unexamined Patent Publication 2000-010072 (FIG. 1, etc.: Patent Document 1) discloses an example of a traditional technique for dealing with such issues. FIG. 23A is a circuit diagram showing an equivalent circuit for one pixel of a liquid crystal display device that is disclosed in Patent Document 1. Explanations will be provided hereinafter by referring to the drawing.
In this technique, transistors for writing video signals to the pixel are two transistors Tr1 and Tr2 which are connected in series. After completing writing of the video signal to the pixel, the two transistors Tr1 and Tr2 are set to be nonconductive simultaneously, and an intermediate node that is a connection point between the two transistors Tr1 and Tr2 is connected via a third transistor Tr3p to a common wiring ST having a voltage that is equivalent to that of a counter electrode. With these operations, out of the two transistors Tr1 and Tr2 which are connected in series, the voltage Vds between the source and drain of the transistor Tr2 that is connected to the pixel becomes irrelevant to the potential of the data line Dm. It is considered therefore to be able to reduce the crosstalk.
Japanese Unexamined Patent Publication 2006-189473 (FIG. 2, etc.: Patent document 2) discloses another example of the traditional technique mentioned above. FIG. 23B is a circuit diagram showing an equivalent circuit for one pixel of a liquid crystal display device disclosed in Patent document 2. Explanation will be provided hereinafter by referring to the drawing.
As in the case of the technique disclosed in Patent Document 1, the transistors for writing a video signal to the pixel are the two transistors Tr1 and Tr2 which are connected in series. It is a method which, after setting the two transistors Tr1 and Tr2 to be nonconductive, connects the intermediate node that is a connection point between the two transistors Tr1 and Tr2 to a common wiring ST having a voltage that is close to the potential of the counter electrode via a third transistor Tr3. With this, out of the two transistors Tr1 and Tr2 which are connected in series, the voltage Vds between the source and drain of the transistor Tr2 that is connected to the pixel becomes irrelevant to the potential of the data line Dm. It is considered therefore to be able to reduce the crosstalk.
The liquid crystal display devices disclosed in Patent Documents 1 and 2 are described by simplifying a part thereof, in order to make clear the differences with respect to the present invention.
However, there are following issues with those traditional techniques.
The first issue is that the manufacturing cost becomes high. With the technique depicted in Patent Document 1, it becomes necessary for the conduction type of the two transistors Tr1, Tr2 connected in series for writing the video signal to the pixels to be different from the conduction type of the third transistor Tr3p for supplying a potential to the intermediate node that is the connection point of the two transistors Tr1 and Tr2. In Patent Document 1, illustrated is a case where the transistors Tr1, Tr2 are n-channel transistors, and the transistor Tr3p is a p-channel transistor. By using the transistors of different conduction types as in this case, it is possible to have a control line (gate line Gn) that is connected to the gate electrodes of the transistors Tr1, Tr2 and a control line (gate line Gn) that is connected to the gate electrode of the transistor Tr3p to be a common line, which makes it possible to control one of the transistors to be conductive and the other to be nonconductive at the same time. With this, it becomes unnecessary to use different control lines for both transistors separately. This is advantageous in terms of improving the numerical aperture of the pixels. However, this requires a process for fabricating the n-channel transistors and p-channel transistors, so that the manufacturing cost is increased.
The second issue is that the numerical aperture becomes deteriorated. With the technique depicted in Patent Document 2, it is possible for the conduction types of all the transistors Tr1-Tr3 used in the pixel to be the same. Thus, the manufacturing cost is not increased. However, it is necessary to control the gate electrodes of the two transistors Tr1 and Tr2 which are connected in series and the gate electrode of the third transistor Tr3 by different control lines. That is, it becomes necessary to provide an additional control line Con for each pixel row for controlling the third transistors Tr3, which results in deteriorating the numerical aperture.